Code Syntax Validator

The banner is a parse, not a blessing

Most syntax pages paint eight languages with one brush. The brush is a regular expression. A regular expression does not know Python's indent stack. A regular expression does not run the PHP tokenizer. This desk refuses to pretend otherwise.

Three modes call a parser the browser already has. JavaScript goes through new Function. JSON goes through JSON.parse. XML goes through DOMParser in text/xml mode. Those three fail for the same class of mistake your runtime fails on: a missing parenthesis, a trailing comma, an unclosed tag.

Python, PHP, CSS, HTML and SQL run a pattern scan. A scan catches a header with no colon, a brace pile that does not return to zero, a SELECT with no FROM in the paste. A scan does not compile. A scan does not execute.

Read the stamp on the rail before you trust the color. Parse means an engine. Scan means patterns. Mixing the two in your head is how a green banner ends up in a pull request description.

Eight modes. Two jobs.

The rail is grouped on purpose. Parse first. Scan second. If you live in one language, a specialist page will beat this desk. The split below is the contract.

new Function is a wrapper. Your file is not.

People paste an ES module, watch the banner go red, and assume the module is broken. The failure is the wrapper.

new Function(source) compiles source as the body of an anonymous function. Function bodies allow return. Function bodies reject import and export. Node loading file.mjs, Vite bundling a component, and a classic <script src> each disagree with this wrapper in a different direction.

import { scan } from './lex.js'
export function run(src) {return scan(src)}

const n = 3
return n * 2

Load the Module (fails here) sample in the JavaScript rail if you want to see the first case without typing. If a real paste starts with import, leave. Run the file under Node, or take it to the JavaScript validator. Debating the banner on this page wastes a stand-up.

TypeScript belongs in the same bucket. const n: number = 1 is legal in tsc and illegal in new Function. There is no TypeScript mode. Strip the types, or compile, before you paste.

Paste the forty lines the stack trace named. A four-thousand-line Laravel controller floods every scanner on the rail. The well is a snippet desk, not a repository host.

Back to the editor if the paste is still sitting in your clipboard.

Counting tags used to punish <img>

An earlier build of this page compared the number of opening tags to the number of closing tags. <img> has no closer. Neither does <br>, <meta>, <input>, or <link>. A valid article full of figures looked unclosed. The scan now skips the void list from the HTML spec. A self-closing <img /> is treated the same way.

The census still cannot see a tag opened inside a Blade @if and closed in a different branch. Comments and <script> bodies still confuse a tally. Server-rendered HTML needs a render of the page, or the dedicated HTML validator, not a tag census.

Same honesty on CSS. Nested &:hover and @layer are grammar your bundler understands. The brace counter here does not. For a stylesheet you intend to ship, the CSS validator is the next hop.

Three greens that still fail on the machine

Syntax is a gate. Runtime, dialect, and unknown properties walk through it every day. These three pastes stay quiet here and then fail where the code runs.

1. Python, a name nobody bound

   def greet():print(user_name)greet()

   The colon is present. The body is indented. The scan has nothing to say. CPython raises NameError: name 'user_name' is not defined the moment you call greet. Syntax was never the problem. Execution was. This desk does not execute.

2. SQL, a Postgres interval on MySQL

   SELECT slug, title
   FROM tools
   WHERE updated_at > NOW() - INTERVAL '7 days'

   SELECT has a FROM. Quotes balance. The scan is quiet. Postgres accepts the interval literal. MySQL wants INTERVAL 7 DAY with no quotes around the number. Dialect is not syntax in the sense this rail measures. Run the statement on the engine you ship to.

3. CSS, a property the browser drops

   .hero-subtitle {colr: #3d4450;max-width: 44rem;}

   Braces balance. Semicolons are present. The scan is quiet. colr is not a CSS error. Unknown properties are ignored. The subtitle stays the wrong color and nobody gets a red banner, here or in DevTools, until someone reads the computed style.

A fourth case belongs next to these even though the banner goes red: JSON with comments. VS Code previews them. JSON.parse refuses them. The comments are not JSON. They are a JSONC habit. Strip them, or keep the file as JSONC and stop calling it JSON. The JSON fixer is built for the trailing-comma variant of the same fight.

Pretty-print is a JSON privilege

The Format control runs JSON.stringify after a successful parse. XML gets a shallow indent if DOMParser accepted the paste. That is the entire formatter.

There is no Black in this tab. No Pint. No php-cs-fixer. No Prettier. A page claiming to reindent Python in the browser without CPython is performing. If the control does nothing, check the rail. Format is silent on JavaScript, Python, PHP, CSS, HTML and SQL on purpose, with a toast that says so, rather than a fake pretty-printer that drifts from what your editor would emit.

Need JSON readable after a minify accident? Stay on JSON and hit Format. Need a package.json with the right keys rather than a pretty bag of guesses? The package.json generator writes the manifest. Need a pattern pulled out of the snippet? The regex tester is the desk for that, not a syntax rail.